A tachometer is used nowadays for measuring and indicating the revolutions of the crankshaft of an engine. Since the engine speed is high, it is necessary to reduce the engine speed in transmitting the revolutions of the engine crankshaft to the tachometer by means of a cable.
To this end, it has hitherto been common practice to use a speed reducing mechanism exclusively provided for the tachometer and interposed between the engine crankshaft and the drive shaft for the tachometer. The use of a speed reducing mechanism of a complex construction has disadvantages in that the mechanism may fail and additionally there is an increase in cost. In order to simplify the construction of the speed reducing mechanism, proposals have been made to use a mechanism wherein the worm mounted at the oil pump shaft is brought into meshing engagement with a tachometer acutating gear by utilizing the oil pump of the engine which is connected to the engine crankshaft through a gear train. However, this entails an undesirable multiplicity of component parts which makes the mechanism complex in construction and increases cost since a speed reducing mechanism used exclusively for the tachometer must be provided.